The present invention relates generally to power supplies for electronically controlled loads and, more particularly, to such a power supply that avoids a large mean rectified AC voltage increase when the load is disconnected, thereby simplifying the control circuit requirements for the electronically controlled load.
In an electronically controlled load, such as of a type supplied by the output of a dimmer circuit in a lamp ballast application, for example, the output load needs to be turned off when the mean rectified AC voltage falls under a safe operating value or under a value for which the circuit will not operate properly. Unfortunately, with no output load connected to the dimmer or rectified AC voltage bus, the mean rectified voltage rises close to the peak value of the input voltage. This makes difficult the design of the voltage sensing and power control circuitry for the electronically controlled load.
Accordingly, it is desirable to provide a practicable solution for avoiding a large mean rectified AC voltage jump and thereby enabling simplification of the voltage sensing and power control circuitry for electronically controlled loads.
In a housekeeping power supply for an electronically controlled load, circuitry is provided for avoiding large increases in the rectified AC bus voltage upon disconnecting the load. The housekeeping power supply for an electronically controlled load is of a type having a semiconductor switch coupled to a rectified AC voltage bus, the switch operating with a Zener diode as a series regulator for providing an output voltage across a resistive voltage divider. The load circuit comprises a current sink, a relatively small energy storage capacitance, and a negative feedback circuit. In an exemplary embodiment, the current sink comprises a resistance coupled to the rectified AC voltage bus for sinking current whenever the semiconductor switch is on. When the semiconductor switch is off, the small capacitance discharges through the resistive voltage divider. When the voltage across the small capacitance decreases to a threshold mean AC rectified voltage bus value, then the negative feedback circuit provides sufficient current to turn the semiconductor switch back on and thus provide approximately the threshold mean AC rectified voltage bus value. The negative feedback circuit time constant, as determined by the small capacitance and the resistive voltage divider, is selected to be sufficiently shorter than the period of the input voltage in order to provide fast feedback response. Diodes are provided on the AC rectified voltage bus in order to ensure fast feedback and to separate the current sink from the remainder of the housekeeping supply during normal circuit operation, i.e., when the electronically controlled load is connected to the supply.